A seal ring is used for a hydraulic machine such as an automatic transmission for a vehicle to prevent oil leakage. The seal ring seals a clearance between a shaft and a housing of a hydraulic machine, in which the shaft is inserted in the housing, for example. In this case, the seal ring is put in a groove of the shaft before inserting the shaft in the housing.
Typically, a seal ring has a rectangular cross-sectional shape (see Patent Literature 1). When hydraulic pressure is applied to such a seal ring, the side surface, which is opposite to the pressure-receiving side surface, comes in contact with the groove of the shaft, and in addition, the outer surface comes in contact with the inner surface of the housing. In this way, the seal ring, which comes in contact with the shaft and the housing, seals the clearance between the shaft and the housing.